Laser welding has gained widespread acceptance in the metalworking industry, producing welds for items ranging from cigarette lighters and watch springs, to medical devices and related components, such as pacemakers, implantable defibrillators, batteries and hybrid circuit packages. Hermetic seals may be provided by laser welding processes to achieve the hermeticity required of implantable medical devices (IMDs) enclosures and associated internal components for IMDs such as batteries and capacitors.
Welding requires heating materials to a molten state so that they become fused together. A laser may be employed to generate light energy that can be concentrated and absorbed at a location in materials, producing the heat energy necessary to perform the welding operation. By using light energy in the visible or infrared portions of the electromagnetic spectrum, energy can be directed from its source to the material to be welded using optics which can focus and direct the energy with the required amount of precision. After the applied light energy is removed, the molten material solidifies and then begins to slowly cool to the temperature of the surrounding material.
At high temperatures, chemical reactions with atmospheric gases (i.e., oxidation) can pose problems, particularly when the oxides or other elements formed have disassociation temperatures far above the melting point of the metal. The result may be brittle, porous welds. A “cover gas,” such as argon or helium (or other inert gas) may be used to cover the welding area, displacing atmospheric gases to minimize the effects of these types of chemical reactions.
The type of weld may have an influence on the laser welding parameters. There are two general weld types—seam welds and spot welds. Seam welding forms a continuous weld, while spot welding consists of discrete weld locations.
Laser welding systems typically consist of a laser source, a beam delivery system, and a workstation. Carbon Dioxide (CO2) and Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) are two laser sources or laser media used for laser welding applications. Both YAG and CO2 lasers may be used for seam welding and spot welding of both butt joints and lap (overlap) joints. Solid state lasers (which includes Nd:YAG, Nd:Glass and similar lasers), are often employed in low- to medium-power applications, such as those needed to spot weld or beam lead weld integrated circuits to thin film interconnecting circuits on a substrate, and similar applications.
For precise or delicate welding operations, solid state welding systems may offer the advantage of coaxial viewing optics that provide magnification so that the exact spot of the laser beam focus can be easily seen. This may enable more precise alignment and focusing of the laser beam, as well as workpiece viewing. Since the wavelength of the Nd:YAG laser is close to the visible spectrum, optical lenses may be used to transmit both the laser light and the image of the workpiece.
In certain welding applications requiring relatively low heat input (due to proximity to thermally-sensitive components, for example), the pulsed laser mode of operation may be suitable. When laser energy is absorbed by the material being welded, heat is conducted into the material, creating a weld pool in a very localized area. Depending on the type of material, some heat may be conducted through the part being welded and away from the weld zone, potentially toward thermally-sensitive material. When using a laser welding system in the pulsed mode, the peak power of the pulsed laser may be much higher than would be delivered in a continuous wave (“CW”) mode, while delivering a lower average power (and hence, less heat) to the component being welded. The higher peak energy may be necessary to create the weld pool, while the lower average power may result in less heat transferred to thermally-sensitive material.
In electronic packaging applications, laser weld sealing can expose internal components, such as heat-sensitive seals, semiconductors, and plastic components, to high temperatures and related thermal effects and stresses. Pulsed Nd:YAG laser welding methods have been used to hermetically seal enclosures in electronic packaging applications. Pulsed laser sources, including Nd:YAG lasers, have been found to be suitable for the welding of electronic packages because the pulsed energy can deliver the necessary power to form a weld on a workpiece, while maintaining relatively low heat input. Pulsed laser welding employs a relatively high peak pulse energy to provide adequate weld penetration, while the intermittent nature of the pulsed energy results in a low average power delivered, which tends to reduce the total heat input.
Certain applications, such as hermetic seam welds, may require a certain amount of overlap (for example, 75-80%) of the laser pulses (successive spot overlap) in order to achieve hermeticity. The required amount of overlap may place constraints on how quickly successive laser pulses may be delivered without heating the device to an unacceptable level. Hence, the amount of time required to complete a hermetic seal weld (i.e., the cycle time) may be lengthened by the successive spot overlap requirement, as well as by the thermal constraints imposed by the presence of thermally-sensitive material.
Pulsed YAG lasers may be used for laser welding of hermetic seals on IMDs and related componentry. Pulsed YAG lasers used for seam welding on IMD components may typically operate at a maximum pulse rate of approximately 10 Hz (10 laser pulses per second), since higher pulse rates could result in unacceptable levels of heating and/or melting of components within an IMD. Components susceptible to heat in an IMD may, for example, include the separator material within the battery of an IMD during the welding of a hermetic seal on the battery housing.
Delivering laser pulses around the periphery of a component to form a hermetic weld seam may require a significant amount of time, depending on such parameters as the length of the weld path, the thermal sensitivity of nearby components, and the need for the laser pulses to overlap sufficiently to create a hermetic seal, as discussed above. Where the pulse rate is limited to 10 Hz, for example, this can result in cycle times of more than 100 seconds to complete a seam weld that is approximately 5 inches long using a spot size of approximately 0.025 inches diameter and an overlap of approximately 80%.
The delivery of laser energy to a particular location on a workpiece may be controlled using a number of different techniques, either individually or in combination. For example, in certain applications, the table or fixture on which the item to be welded (i.e., the workpiece) is disposed may be moved in two dimensions with respect to a laser beam that is stationary. This technique is sometimes referred to as an “x-y table” system. Such systems are typically used in applications requiring relatively slow movement of the workpiece with respect to the laser beam.
The amount of time required to perform certain welding operations, such as hermetic seal welds in IMDs and related componentry, imposes a constraint on manufacturing such devices due to the need to maintain a relatively low heat input to the device, as well as the need to achieve the degree of weld overlap necessary to achieve sufficient hermeticity. Additionally, the physical size of devices such as IMDs is constrained to a certain degree by the need to provide thermal barriers/shields to protect components within IMDs from the heat generated during the laser welding process.